When i flex my back/traps, get a pain in my shoulder blade mostly, and towards the shoulder area, what could it be?

Shoulder pain: Sounds muscular likely due to spasm, and possibly a referred pain pattern from trigger points . Trigger points are hyper irritable knots or areas of muscle spasm. They are generally tender and sometimes produce a local twitch reaction when palpated or needled. Each muscle has their own referral pattern. Read up on janet travell's work on this as she is most famously known as jfk's physician.